how to post a complaint and this is my complaint

I've decided to leave ATT transferring my  home phone service on October 18th, 2016. That's 6 months ago ....and I'm still being billed for the telephone number.  I've been going to their shop in Cherry Creek Denver.  Two times, two associates assisted my in calling their own customer service to make sure my message was clear to that department and it hasn't helped.  Last month I got tired so tired of paying for the non-service I withheld payment.... on the next billing they charged me AGAIN !!  new charge plus last months charge plus a late fee penalty for a number that I transferred to Comcast:  the lingo is provisioned , ported ....whatever, can't get ATT to understand,  I'm paying for my telephone number at ATT and my selected new carrier, Comcast.  Double dipping on me .... Today is 28 April 2017, AND I'm STILL having to pay my bill for a feature I don't have.  They've sent me to Comcast, Comcast sends me back to ATT and so on and so for.... It is no fun playing ping pong with my money.

Any suggestions how and who I can register this complaint too?  Posting on Formal complaints to FCC now...